1. Field of the Disclosure
The present disclosure relates generally to image forming devices and more particularly to a developer roll having magnetic zones of varying axial length for a dual component development electrophotographic image forming device.
2. Description of the Related Art
Dual component development electrophotographic image forming devices include one or more reservoirs that store a mixture of toner and magnetic carrier beads (the “developer mix”). Toner is electrostatically attracted to the carrier beads as a result of triboelectric interaction between the toner and the carrier beads. A developer roll includes a stationary core having one or more permanent magnets and a sleeve that rotates around the core. The permanent magnet(s) produce a series of magnetic poles that are circumferentially spaced around the outer surface of the sleeve. The magnetic poles attract the carrier beads in the reservoir having toner thereon to the outer surface of the sleeve, which transports the developer mix as the sleeve rotates. A photoconductive drum is charged by a charge roll to a predetermined voltage and a laser selectively discharges areas on the surface of the photoconductive drum to form a latent image on the surface of the photoconductive drum. The sleeve of the developer roll carries the developer mix in close proximity to the photoconductive drum. The sleeve is electrically biased to facilitate the transfer of toner from the chains of developer mix on the outer surface of the sleeve to the discharged areas on the surface of the photoconductive drum forming a toner image on the surface of the photoconductive drum. The photoconductive drum then transfers the toner image, directly or indirectly, to a media sheet forming a printed image on the media sheet. Developer mix on the outer surface of the sleeve that is not transferred to the photoconductive drum is transported by the sleeve back to the reservoir. After the remaining developer mix reenters the reservoir, the developer mix is no longer magnetically retained against the outer surface of the sleeve allowing the developer mix to release from the sleeve back into the reservoir.
In general, the sleeve of the developer roll has a greater axial length than the core such that axial end portions of the sleeve extend past both axial ends of the core. The magnetic field lines from the core extend past the axial ends of the core and attract fine amounts of developer mix to the surface of the sleeve past the axial ends of the core. Developer mix on the surface of the sleeve past the axial ends of the core is generally not dense enough to form full quality images on the surface of the photoconductive drum. Accordingly, transfer of toner from the developer mix on the surface of the sleeve past the axial ends of the core to the surface of the photoconductive drum at the outer axial portions of the photoconductive drum is undesired.
The presence of unwanted developer mix on the surface of the sleeve past the axial ends of the core also increases the risk of leakage of developer mix from the system. During operation, developer mix may tend to accumulate on the outer axial end portions of the sleeve and leak past the axial ends of the sleeve potentially contaminating other parts of the system.
One method to reduce the unwanted transfer of toner from the surface of the sleeve past the axial ends of the core to the surface of the photoconductive drum includes extending the length of the charge roll in order to charge the surface of the photoconductive drum at the outer axial ends of the photoconductive drum past the axial ends of the core to a voltage that will resist the charged toner. However, increasing the length of the charge roll does not address the leakage risk and may increase the size and cost of the system.
Another method to reduce the unwanted transfer of toner from the surface of the sleeve past the axial ends of the core to the surface of the photoconductive drum includes placing a magnetic shunt axially outboard of each axial end of the core. Each magnetic shunt is composed of a magnetically permeable metal that redirects the magnetic field lines from the axial ends of the core back into the core to decrease the distance that the magnetic field lines extend axially past the core. As a result, the magnetic shunts decrease the distance the developer mix on the surface of the sleeve extends past the axial ends of the core thereby reducing the required length of the charge roll. However, the magnetic shunts may not sufficiently address the leakage risk.
One method to reduce leakage of developer mix at the axial ends of the sleeve includes positioning a magnetic seal in close proximity to the surface of the sleeve axially outboard of each magnetic shunt to capture any developer mix that leaks axially outward past the magnetic shunts. The magnetic seals are composed of permanent magnets that attract the developer mix to the seals. The magnetic seals must be positioned far enough axially outboard from the developer mix that is released from the sleeve back into the reservoir, otherwise the magnetic seals can become contaminated with the released developer mix limiting their sealing effectiveness. As a result, the magnetic seals may increase the size of the system.
Accordingly, an improved method to reduce the amount of carrier beads and toner on the surface of the sleeve of a developer roll past the axial ends of the core of the developer roll and to reduce developer mix leakage while minimizing the size of the system is desired.